Cementless, porous coated prostheses are being used widely as attractive alternatives to cemented components in total joint arthroplasty. However, inadequate initial stability, inadequate bony ingrowth into the porous coatings, early loosening, and persistent pain have been identified as major problems with these devices. The difficulties involved in accurately machining the bone surfaces with manually operated instruments have been shown to be responsible for many of these problems. The efficacy of using a highly sophisticated and precise robotic instrumentation system for preparing the femoral canals accurately to accept the prostheses and thus improve the initial stability and long term fixation by bone ingrowth will be investigated in this study. Total hip arthroplasties will be carried out by using conventional and robotic instrumentation in cadaver femurs of dogs and humans and the prosthetic stability will be evaluated in vitro under loads simulating the midstance of dogs and humans. In order to evaluate the efficacy of the preparation on bone ingrowth, total hip arthroplasties will also be carried out in vivo in dogs using the same techniques, and the dogs will be sacrificed after a three month observation period. The femurs will be sectioned and subjected to detailed histomorphometric evaluations to determine the tissue ingrowth. The data from this study should provide valuable information on the potential for extending the longevity of cementless prostheses by using the robotic instrumentation for preparing the bone surfaces accurately and precisely.